The demand for so called dual mode terminals or stations is expected to increase with time. In the United States of America there is presently at least one system which supports dual mode operation, i.e., digital TDMA and analog FM, also referred to as DAMPS. As the systems such as GSM, PCS, DECT, DCS1800 and TDMA1900 become more widely used there is expected to be a need to have mobile stations which support two modes of operation, such as GSM and DCS1800 or DAMPS and TDMA1900 or DCS1900. A proposed third generation mobile telecommunication systems, such as UMTS (in ETSI) or FPLMTS (in CCIR), is under development. These advanced systems may also require some type of dual mode operation.
In an article entitled "GSM and DECT-A Dual Mode Solution", Mobile Communications International Apr. 21, 1995, pgs. 57-60, B. Rashidzadel et al. describe an RF subsystem in FIG. 3 for a dual mode handset that operates in the GSM frequency band (890-960 MHz) and the DECT frequency band (1880-1900 MHz). In the transmitter portion a single I/Q modulator provides direct modulation at either the DECT or GSM frequency bands, and is connected through a SPDT switch to one of a DECT or a GSM transmitter chain. In the receiver portion separate DECT and GSM low noise amplifiers (LNAs) are used due to the large difference in frequencies. Following the GSM LNA and an image reject filter, the next stage is a dual stage downconversion device implemented as an integrated circuit chipset, e.g. Siemens PMB2402. In the DECT receiver, a number of currently available chipsets which employ an LNA and first downconversion stage are said to be suitable for use. A SPDT switch is employed to select one of the GSM or DECT receive chains for inputting to the last downconverter stage of the dual stage downconversion device. In this approach the second stage of the downconversion chip is common to both the GSM and DECT receive chains, which is said to result in a common second IF signal which can be digitally sampled, digitally filtered (depending on the mode of operation), and suitably demodulated.
In this approach a total of at least three local oscillator signals are required, one for the DECT downconverter, and two for the dual stage GSM, GSM/DECT downconverter. Also in this approach both the GSM and the DECT chains employ a double conversion superheterodyne receiver.
In commonly assigned Finnish Patent Application FI 961465 a dual band transmitter/receiver is disclosed that is based on direct conversion to baseband, wherein a single frequency synthesizer is used for both bands. When operating in the higher frequency band the output of the synthesizer is employed as the mixing frequency, while in the lower frequency band a mixing frequency is obtained by dividing the output frequency of the synthesizer by at least two.